Mesocrates rock

Gabbros with an average mineral inventory belong to the mesocratic rocks. (Boulder of a Precambrian gabbro from Scandinavia: the darker areas show fresh rock, the lighter surfaces are weathered.)

Mesocrate rocks (from ancient Greek μέσος mésos "medium"), also known as mesotype rocks , are igneous rocks with neither a particularly light nor a particularly dark shade. The reason for this is that the rock has a volume fraction of dark mafic minerals ( color index M ′ ) that is in the middle range with 35 to 65%. Rocks with a lower M ′ (<35) are called leucocrat and rocks with a higher M ′ (> 65) are called melanocrat . Typical mesocrate rocks are e.g. B. diorite or monzonite .

This use of the attribute “mesocrat” for rock characterization is rarely found in the specialist literature. As a rule, a distinction is only made between leuocrat and melanocrat magmatites, often only based on a purely visual impression, without the exact proportion of mafic minerals being known.

literature

• RW Le Maitre (Eds.), A. Fahrtisen, B. Zanettin, MJ Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efermova, J. Keller, J. Lameyre, PA Sabine , R. Schmid, H. Sørensen, AR Woolley: Igneous Rocks: A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks. Cambridge University Press, 2002, ISBN 978-0-521-66215-4
• Wolfhardt Wimmenauer: Petrography of igneous and metamorphic rocks. Enke-Verlag, Stuttgart 1985, p. 106 ff., ISBN 3-432-94671-6

Remarks

1. ↑ Usually, a volume fraction is always given in percent. However, if the volume fraction of dark Mafite is specified as the color index M ′, the percentage symbol is omitted.